Ash formation during industrial-scale bubbling fluidized bed (BFB) and circulating fluidized-bed (CFB) combustion of bark, waste wood and paper mill sludges has been studied. The principal ash formation mechanism was the sintering of 0.2-4 mu m paper filler mineral particles into larger, porous ash agglomerates From 10 to 200 mu m in size which dominated the fly sh total mass and specific surface area. Fly ash total mass concentration at the electrostatic precipitator inlet conditions varied from 4.5 g N(-1)m(-3) at BFB to 8.3 g N(-1)m(-3) at CFB while the respective total surface area concentrations as determined with nitrogen absorption were 32 and 83 m(2) N(-1)m(-3). During bark combustion in the BFB, we observed an ultrafine mode at 0.05 mu m consisting of spherical particles and their chain-type agglomerates. Ultrafine mode mass and surface area concentrations were 3 and 0.1 m(2) N(-1)m(-3), respectively, as determined with the electrical low-pressure impactor and with the differential mobility analyzer. Ultrafine particles were formed via nucleation of volatilized ash species followed by particle growth via collision and coalescence and via vapor condensation. During waste wood combustion in the CFB, ultrafine particle concentration was significantly lower than during bark combustion in BFB, indicating reduced ash volatilization during CFB combustion. When co-firing sludge with bark in the BFB end with waste wood in the CFB ultrafine particle concentration was Further reduced. Results from continuous aerosol measurements by the electrical low-pressure impactor (ELPI), by the scanning differential mobility analyzer (SMPS) and by the tapered element oscillating microbalance (TEOM) as the function of BFB process conditions suggest that co-combustion of sludge reduces significantly the ultrafine particle formation rate. No significant enrichment of alkali and trace metals in the ultrafine particles was observed.